The authors (including Jared Diamond, who communicated the paper to PNAS), set out to determine why there are so many species of white-eyes, a group of songbirds distributed across Africa, Southeast Asia, and the southern Pacific. They built a phylogeny for the group, calibrated it to real time using the geological dates of origin for Pacific islands occupied by white-eyes, and then estimated the rate at which the group produced new species. They found, as reported by Wired.com, that the largest group of white-eyes have one of the fastest species-accumulation rates recorded in vertebrates, about 1.6 new species every million years.
Photo by Lip Kee.
Photo by Lip Kee.
That's a weird result, when you think about it - we're talking about birds, and widely-distributed birds, here. All things being equal, speciation is facilitated by lack of movement - Appalachian salamanders, for instance, diversified largely because they're too gimpy to move between stream drainages very often [$-a]. Furthermore, the authors say, white-eyes don't display a lot of ecological differences that might contribute to isolation. So how did they speciate at a record-setting pace?
The solution? The authors propose that white-eyes are prone to rapid changes in their dispersal ability. As evidence, they cite numerous cases in which white-eyes must have crossed great distances to colonize one island, then failed to make it across much smaller distances to colonize others nearby. Nodding to Diamond's groundbreaking work on human history and cultural evolution, they compare this to the colonization of Polynesia, in which people stopped traveling long distances as the chance of discovering an uninhabited island decreased.
K.H. Kozak, D.W. Weisrock, A. Larson (2006). Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon) Proceedings of the Royal Society B: Biological Sciences, 273 (1586), 539-46 DOI: 10.1098/rspb.2005.3326
R.G. Moyle, C.E. Filardi, C.E. Smith, J. Diamond (2009). Explosive Pleistocene diversification and hemispheric expansion of a "great speciator" Proceedings of the National Academy of Sciences, 106 (6), 1863-8 DOI: 10.1073/pnas.0809861105